Polyalkylene terephthalate resins, such as polybutylene terephthalate (hereinafter sometimes referred to by the abbreviation "PBT"), are well known crystalline thermoplastic resins which exhibit excellent mechanical, electrical, chemical and physical properties. As a result, polyalkylene terephthalate resins have been widely used as an engineering plastic in a variety of end-use applications, including component parts for automobiles, electrical and electronic appliances, and the like. PBT is also known to exhibit excellent aroma retention and gas-barrier properties and thus has been used recently as a food and cosmetic packaging material.
However, there exist several problems when polyalkylene terephthalate resins are molded. Significantly, when polyalkylene terephthalate resins are melt-processed, they often decompose and thus generate decomposition gases and/or decomposition residues (sublimate) which adversely affects the surface quality of molded articles (e.g., by formation of voids on the surface) as well as staining/corroding the mold from which the article is made. Such decomposition of polyalkylene terephthalate or polyalkylene naphthalate resins is known to be exacerbated by intermittent (rather than continuous) molding operations which require that the resin be maintained in a molten state for a prolonged time period.
Furthermore, problems associated with polyalkylene terephthalate resins in terms of evolution of a decomposition gas (e.g., tetrahydrofuran) have been experienced when molded articles are used in an environment where the part is exposed to elevated temperatures for prolonged time periods. Accordingly, an electrical or electronic component (such as a relay, switch or connector) which is comprised of an article molded from a conventional polyalkylene resin is problematic in that the evolved decomposition gas will stain or corrode metallic parts of the component (e.g., electrical contacts) and thus reduce the component useful life.
What has been needed therefore is a polyalkylene terephthalate resin composition which is highly melt-stable to an extent whereby evolved decomposition products during melt-processing and/or during which temperature use is significantly minimized (if not eliminated entirely). It is towards fulfilling such a need that the present invention is directed.
According to the present invention, modified polyester resins are provided which emit a minimal amount of decomposition gas (such as tetrahydrofuran) when used at high temperatures for prolonged time periods, as well as a decreased amount of sublimate when in the melt-phase. More specifically, the present invention relates to modified polyester resins which are the reaction product of:
(A) an acid component mainly comprised of terephthalic acid, 2,6-naphthalene dicarboxylic acid or a lower alcohol ester thereof,
(B) a diol component mainly comprising an alkylene glycol having 2 to 8 carbon atoms, and
(C) an aromatic sulfonic acid compound of the following general formula (1): EQU HO(RO).sub.n --Ar--SO.sub.3 M (1)
wherein Ar represents a benzene or naphthale ring, n represents an integer from 1 to 10, R represents a group selected from among alkylenes having 2 or 3 carbon atoms and when n is 2 or more, R may be the same as or different from one another, and M represents an alkali metal selected from lithium, sodium and potassium.
Most preferably, the modified polyesters of this invention will also satisfy the following formulas (2) and (3): EQU 0.25.ltoreq.x.ltoreq.100(meq/kg) (2) EQU 2.ltoreq.(x/CEG).times.100.ltoreq.150 (%) (3)
where x represents the amount of the aromatic sulfonic acid compound represented by the above general formula (1) introduced thereinto and CEG represents the amount of the carboxy end groups, and x and CEG are each expressed in milliequivalent per kilogram (meq/kg) of the polymer.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.